The present inventive concept relates generally to semiconductor memory devices, and more particularly, to non-volatile memory devices having reduced bit line bias time.
Semiconductor memory devices include volatile semiconductor memory devices and non-volatile semiconductor memory devices. Volatile semiconductor memory devices are characterized by fast read/write speeds, but lose stored data in the absence of applied power. In contrast, non-volatile semiconductor memory devices have relatively slower read/write speeds, but are able to retain stored data in the absence of applied power. Thus, contemporary non-volatile semiconductor memory devices are commonly used in a variety of applications (e.g., portable electronic devices) benefiting from the secure retention of stored data even when power is interrupted, or only intermittently applied.
Nonvolatile semiconductor memory devices include phase-change random access memory (PRAM), mask read-only memory (MROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), and electrically erasable programmable read-only memory (EEPROM). The MROM, PROM, and EPROM among other types of nonvolatile semiconductor memory devices are difficult to program and erase using methods commonly accessible to the end user. On the other hand, EEPROM uses electrical program and erase operations readily controllable by the end user. As a result of this programming ease of use, the EEPROM has become a common data storage medium for both system programming data, as well as payload (or user) data. EEPROM finds application as a system memory or as an auxiliary memory.
Flash memory is one particularly useful form of EEPROM. Because of its high degree of integration, flash memory is very advantageous when used as a large-capacity auxiliary memory device. Accordingly, the flash memory is widely used in small electronic devices, such as the digital camera, digital camcorder, digital music player, etc., where data storage must be accomplished by circuitry having a compact size and competent reprogramming capabilities. Flash memory includes NAND-type flash memory and NOR-type flash memory.
Data is typically accessed in flash memory using a program operation, an erase operation, and a read operation. Before the read operation is executed, a precharge operation is performed. As part of the overall precharge operation, a bit line precharge operation is performed, wherein a bit line is biased by the application of electrical charge until the bit line reaches a predetermined precharge voltage.
Time required to precharge the bit line (hereafter referred to as the “bit line precharge time”) is proportional to the resistance and the capacitance of the bit line. As the length of the bit line increases, its resistance and capacitance also increase. Accordingly, longer bit lines generally require a longer bit line precharge time. However, any increase in the bit line precharge time causes read operation performance (as measured e.g., in Megabytes (MB) per second) to deteriorate. Program operation performance is also adversely affected. Therefore, reducing the bit line bias time required by a nonvolatile memory device increases overall performance.